Method of and means for reducing triboelectric forces in electrophotography



Dec. 2, 1958 A. J. MONCRlEFF-YEATES 2,862,816

METHOD OF AND MEANS FOR REDUCING TRIBOELECTRIC v FORCES IN ELECTROPHOTOGRAPHY Filed March 26, 1954 M77961? 1 50am:

m J Jami BY ATTORNEY METHOD OF AND MEANS 'FOR REDUCING TRIBQELECTRIIC FGRES 1N ELECTROPHO- TQGRAPHY Alexander J. Moncrieif-Yeates, Cranbury, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application March 26, 1954, Serial No. 419,060

3 Claims. (Cl. 961) This invention relates to methods of and means for reducing triboelectric forces existing between two dissimilar bodies in surface contact with one another with particular applications to electrostatic printing processes.

It has been noted that, when'two bodies of different materials are in surface contact with one another, an attractive or triboelectric force usually exists between thebodies. When it is desired to separate these bodies, this attractive or triboelectric force must be overcome. A triboe'lectric force is primarily the electrostatic attractive force'due to a contact potential existing between two bodies. Generally speaking, it is believed that contact potential is a function of the difference between the work functions of two materials. When surfaces of the two bodies are in contact with each other, it is believed that electrons from one body pass into the other until the potential between the two bodies in contact becomes zero. librium state, one body is positively charged with'respect to the other, and therefore there exists 'an electrostatic attractive force between the bodies. The greater the contact potential between these bodies, the greater the triboelectric force.

Ordinarily triboelectric forces are small when the masses of the bodies are-relatively large. However, in certain cases, for example, where there are fine particles, these forces are large relative to other forces involved. One application wherein these forces may be important is in electrostatic printing processes.

An electrostatic printing process'is, generally speaking, that type of process for producing a visible record, reproduction or copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on a dielectricbase. The process may also include the conversion ,of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be of a different size.

In one type of electrostatic printing process, more fully described in U. S. patent application Serial No. 389,347, filed October 30, 1953, by A. J. 'Moncrietf- Yeates, an electrostatic image is established upon the surface of a dielectric image sheet. A charged developer powder substance is then brought into close spaced relation with the electrostatic imageand an electric field is applied to the charged developer .powder, whereby the developer powder is attracted to and deposits upon the electrostatic image.

According to one embodiment, the developerpowder is charged by applying an electric field to a layer of I powder resting on a lower electrode whereby the powder becomes charged and is repelled from the lower electrode. The repelled charged powder may be collected upon a suitable surface. The electrostatic image is brought into close spaced relation with the charged powder and then an electric field is applied to the developer powder whereby the devoloper powder is at tracted to and deposits upon the electrostatic image.

in this embodiment the developer powder, resting upon In this equiited States Patent p 2,862,816 Patented Dec. 2, 71958 a lower electrode, is charged by applying an electric field thereto. In order to repel the powder from the electrode, not only must gravitational forces be overcome, but triboelectric forces existing between the developer powder and the electrode must also be overcome. In the second step, after the charged powder has been collected upon a suitable surface, an electrostatic image is brought into close spaced relation with the charged powder and an electric field is applied ,in order to form the electrostatic powder image. Here again, triboelectric forces existing between the powder and the surface must be overcome in order for the powder particles to pass from the surface to the electrostatic image.

An object of the invention is to provide methods and means for reducing triboelectric forces existing between bodies of materials in surface contact with one another.

A further object of the invention is to provide 'improved methods and means of electrostatic printing, and printing materials therefore.

Another object is to provide improved methods and means for developing electrostatic images which require substantially lower voltages resulting in increased sensitivity.

Another object is to materially reduce the triboelectric forces between particles of a mixture comprising particles of materials separated from one another in the tribeelectric series.

The foregoing objects and other advantages may be accomplished in accordance with the present invention which comprises a method of reducing the attractive force existing between a first body and a second body in surface contact with each other, which comprises providing on the con-tact surface .of said first body, a coating of a material having a triboelectric value, approximately the same as the triboelectric value of the material of the second body.

The foregoing objects and other advantages will be more apparent and the invention will be described in greater detail by reference to the accompanying drawing of which:

Figure 1 is a partially schematic sectional view of an apparatus for charging a developer powder by a method of this invention.

Figure 2 is a partially schematic sectional view of an apparatus for developing an electrostatic image using the charged powder of Figure 1, and

Figure 3 is a partially schematic sectional view of an apparatus for simultaneously forming an electrostatic image, charging a developer powder and developing the electrostatic image with the charged developer powder.

Similar reference characters are applied to similar elements throughout the drawing.

Example 1 Referring to Figure 1, a lower electrode comprises a brass plate 21 held at ground potential through a lead 47. A thin layer 51 of polyvinyl formal resin of the order of 3 to 7 microns thick is coated on the upper surface of the brass plate 21. A thin layer 31 of polyvinyl formal resin developer powder rests upon the upper surface of the coating 51 of polyvinyl formal. An upper electrode comprises a brass plate 25 having a polyvinyl formal coating 23 on its lower face. The brass plate 25 of the upper electrode isheld at a negative potential-of about 300 volts with respect to ground .by a voltage source 53 connected thereto through a lead 45, a voltage dividing potentiometer '61 and a double-pole, doublethrow reversing switch 63. The upper electrode .25 is brought parallel to and about inch away from the powder layer 31 on the lower electrode. The brass plates 21 and 25 of the electrodes, acting as plates ofa Condenser, charge the powder particles 31 positively. The charged powder particles are attracted upwards through the electric field between the electrodes, depositing as a thin layer 33 of charged particles on the lower surface of the polyvinyl formal coating 23.

In order for the powder particles 31 lying upon the lower electrode to become charged, the charge must pass from the brass plate 21 through the polyvinyl formal layer 51 into the powder particles. Polyvinyl formal is ordinarily a dielectric and one would expect that charges would not pass through this layer. However, it has been found that thin layers of most dielectrics, including polyvinyl formal, will behave as if they were semiconductors when electric fields are applied. A layer 51 of polyvinyl formal of the order of 3 to 7 microns thick passes suflicient electric charges to enable powder particles 31 lying thereon to become charged.

Polyvinyl formal powder particles and the brass electrode ordinarily develop substantial triboelectric forces between them when they are in surface contact with each other. Substantial additional electric fields are required to move charged polyvinyl formal particles from a brass surface in order to overcome these triboelectric forces. According to the invention, by providing on the brass plate a surface of a material having a triboelectric value approximately the same as the triboelectric value of the polyvinyl formal powder, these triboelectric forces are substantially reduced. The coating 51 of polyvinyl formal upon the brass plate reduces this force by providing a contact surface of the same material as the developer powder. By selecting the coating less than 7 microns thick, electric charges are permitted to pass through resulting in a substantial reduction of the total voltage required. If the polyvinyl formal powder were in direct contact with the brass surface, about 600 volts or 4800 volts per inch is required, in the above example. The polyvinyl formal coating 51 on the brass surface allows a voltage reduction by a factor of about 2, to about 300 volts or 2400 volts per inch, to accomplish the same result. This effect is most pronounced when at least one of the bodies in contact has a resistivity greater than 10 ohm-centimeters.

When a sufiiciently thick layer of charged particles 33 has formed, which is of the order of 12 layers of particles, the lower electrode is withdrawn. Referring to Figure 2, a second lower electrode comprises a brass plate 27 and having on its upper surface a dielectric layer 29 having an electrostatic image established thereon. The electrostatic image may be established, for example, by providing a dielectric layer 29 that is also photoconductive. Dielectric photoconductive layers may be of materials such as photoconductive selenium, anthracene, cadmium sulphide or mixtures of photoconductors in dielectric film-forming resins such as polystyrene or silicone resins. The surface of the photoconductive layer is given a blanket negative charge, for example, from a corona discharge apparatus and then exposed to an optical image. The negative charge in the illuminated areas of the photoconductive layer is discharged, leaving a negative charge in the non-illuminated areas of the surface of the photoconductive layer, thereby producing an electrostatic image on the surface of the photoconductive layer.

The brass plate 27 is grounded through a lead 49 and brought parallel to and within about A; inch of the layer of charged particles 33. A positive voltage of about 50 volts with respect to ground is applied to the upper electrode. On bringing the electrostatic image close to the charged particles 33 and applying the positive voltage, the combination of (1) the electric field formed by the voltage applied between the upper and lower electrodes, and (2) the attraction of the positively-charged developer powder for the negatively-charged areas of the electrostatic image, causes the positively-charged developer powder 33 to be attracted to and deposit upon the negatively-charged areas of the electrostatic image, thereby producing a powder image 35.

Triboelectric forces exist between the charged powder 33 upon the upper electrode and the electrode material. Here again the triboelectric forces must be overcome by additional electric fields. In the case of charged powders in direct contact with the upper brass plate 25, positive voltages of about volts or 800 volts per inch are required to obtain satisfactory results. By interposing a Example II Referring to Figure 3, it is sometimes desirable to establish an electrostatic image, to charge the developer powder and to develop the electrostatic image in a single operation. The apparatus of Figure 3 is similar to the apparatus of Figure 1, except that the upper electrode comprises a sheet of glass 26, a transparent electricallyconducting coating 27 such as Nesa, marketed by the Pittsburgh Plate Glass Co., Pittsburgh, Pa., on the lower surface thereof and a layer of a dielectric photoconductive material 29', for example, photoconductive selenium thereon. The lower electrode comprises a brass plate 21 held at ground potential through a lead 47. A thin layer 51 of polyvinyl formal is coated on the upper surface of the brass plate 21. A thin layer 31 of polyvinyl formal powder rests upon the upper surface of the thin layer 51. A positive voltage with respect to ground is applied to the electrically conductive layer 27 of the upper electrode and an optical image is projected from above through the conductive layer 27 onto the photoconductive layer 29'. The illuminated areas of the photoconductive layer 29' forms a positive electrostatic image on the lower surface of the photoconductive layer 29'. Simultaneously a negative charge, attracted to the lower electrode due to the capacitance of the apparatus charges the developer powder negatively. The negatively-charged particles 31, attracted by the electric field between electrodes and the electrostatic image, move upwards and deposit upon the positively-charged areas of the electrostatic image on the photoconductive layer 29, thereby forming a powder image.

As in Example 1, triboelectric forces exist between the powder particles 31 lying upon the lower electrode. When polyvinyl formal powder rests directly upon a brass surface, additional electric fields are required to overcome the triboelectric forces. However, by interposing a layer of polyvinyl formal up to 7 microns thick, a reduction in voltage by a factor of about 2 is obtained. This reduction is due to the fact that relatively small triboelectric forces exists between the powder and the polyvinyl formal layer and because polyvinyl formal layers up to 7 microns thick will conduct sufficient amounts of electric charge.

Examples of other embodiments in which the invention effects substantial reductions in triboelectric forces are as follows:

(1) Polystyrene powder lying on a brass plate requires fields of 35 kilovolts per inch to be attracted away. Coating the brass plate with a thin layer of polystyrene allows the use of electric fields of the order of 12 kilovolts per inch.

(2) Brass powder lying on a layer of illuminated photoconductive selenium requires fields of about 20 kilovolts per inch to be attracted away. Coating a layer of indium a fraction of an Angstrom unit thick allows the use of electric fields of the order of 7 kilovolts per inch without affecting the storage properties of the selenium.

(3) Vinyl chloride powder in contact with a brass plate requires relatively high electric fields to separate the two. Substantial reductions in triboelectric forces are effected by coating the brass plate with vinyl chloride resin in a layer up to one micron thick.

(4) A field of more than 2500 volts per inch is required to separate a. polyvinyl formal resin powder from a copper plate. Only about 1000 volts per inch is required if the copper plate has a coating of polyvinyl formal on the surface in contact with the powder.

(5) Where a developer mix comprises a mixture of developer powder particles and granular carrier particles, the triboelectric force existing between the developer powder particles and the carrier particles may be substantially reduced by coating the carrier particles with a thin coating of the material of the developer powder or some other material having a position in the triboelectric series close to the position of the developer powder material.

The above-described examples relate to electrostatic printing and are particularly concerned with the reduction of the physical forces required to separate two dissimilar materials. The reduction of this force is effected by reducing the electrostatic attraction that arises between the two bodies due to differences in contact potential.

The invention may also be applied to the reduction of static electricity when a body of one material rubs against another material. For example, the invention may be used in the textile field to reduce the static electricity that is generated (1) when synthetic yarn passes through yarn guides, (2) when a person slides across automobile seat covers that are made of a synthetic material, or (3) when synthetic clothing, as raincoats or other garments, are brushed across and stick to adjacent surfaces due to static electricity.

There have been described methods and means for reducing triboelectric forces existing between two bodies by interposing a layer of a material having a triboelectric value approximately the same as the triboelectric value of the material of one of the bodies. In certain electrostatic printing processes, substantial reductions in the electric fields may be obtained by the method of the invention.

What is claimed is:

1. A method of rendering visible an electrostatic image with a developer powder which comprises positioning the electrostatic image between an upper electrode and a lower electrode having on its upper surface a coating of a material comprising a continuous film integral with said surface, said material having a position in the triboelectric series substantially the same as the position of the developer powder material and having a thin layer of said developer powder resting upon the upper surface 6 of said coating and applying a voltage between the electrodes whereby the powder becomes charged and is attracted to and deposits upon the electrostatic image producing a visible image.

2. An electrostatic printing device of the type employing electrostatically attractable developer powder comprising a first electrode and a closely spaced second electrode, said first electrode having a coating of the order of 3 to 7 microns thick upon its inner surface to form thereon a continuous .film integral with said inner surface, said coating consisting of a material having a position in the triboelectric series substantially the same as the position of said developer powder and said second electrode having a dielectric photoconductive coating upon its inner surface, means for introducing an optical image upon said photoconductive coating and connection means for a source of voltage for producing a potential difference between said electrodes.

3. A method of electrostatic printing comprising providing an upper member including a transparent first electrode and a photoconductive layer coated on the lower surface of said first electrode, closely spacing from said upper member a lower member including a second electrode, a thin layer of an insulating material coated on the upper surface of said second electrode to form thereon a continuous film integral with said upper surface and a thin layer of developer powder superimposed upon said coating, said insulating material and said developer powder having substantially equal triboelectric values, projecting a light image upon said photoconductive layer, simultaneously applying an electric field between said electrodes whereby said developer powder becomes charged and is attracted to and deposits upon an electrostatic image formed in said photoconductive layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,233,037 Smith Feb. 25, 1941 2,248,064 Carlton et al. July 8, 1941 2,277,013 Carlson Mar. 17, 1942 2,297,691 Carlson Oct. 6, 1942 2,316,824 Tobias et al. Apr. 20, 1943 2,618,551 Walkup Nov. 18, 1952 2,618,552 Wise Nov. 18, 1952 2,633,796 Pethick Apr. 7, 1953 2,659,670 Copley Nov. 17, 1953 2,701,764 Carlson Feb. 8, 1955 2,752,833 Jacob July 3, 1956 2,756,676 Steinhilper July 31, 1956 2,803,177 Lowrie Aug. 20, 1957 2,808,328 Jacob Oct. 1, 1957 FOREIGN PATENTS 16,287 Great Britain of 1910 

1. A METHOD OF RENDERING VISIBLE AN ELECTROSTATIC IMAGE WITH A DEVELOPER POWDER WHICH COMPRISES POSITIONING THE ELECTROSTATIC IMAGE BETWEEN AN UPPER ELECTRODE AND A LOWER ELECTRODE HAVING ON ITS UPPER SURFACE A COSTING OF A MATERIAL COMPRISING A CONTINUOUS FILM INTEGRAL WITH SAID SURFACE, SAID MATERIAL HAVING A POSITION IN THE TRIBOELECTRIC SERIES SUBSTANTIALLY THE SAME AS THE POSITION OF THE DEVELOPER POWDER MATERIAL AND HAVING A THIN LAYER OF SAID DEVELOPER POWDER RESTING UPON THE UPPER SURFACE OF SAID COATING AND APPLYING A VOLTAGE BETWEEN THE ELECTRODES WHEREBY THE POWDER BECOMES CHARGED AND IS ATTRACTED TO AND DEPOSITS UPON THE ELECTROSTATIC IMAGE PRODUCING A VISIBLE IMAGE. 